Emitters are known of the type comprising at least one loudspeaker mounted in the wall of a speaker enclosure with one face of the diaphragm radiating into the outside air and the other face radiating into the inside of the enclosure.
The enclosure is generally provided with a vent putting the inside of the enclosure into communication with the outside, and enabling pressure waves generated by the diaphragm and radiating into the enclosure to be radiated outwards.
The vent forms the orifice of a Helmholtz resonator whose cavity is constituted by the enclosure. In application of a well-known property of that type of resonator, the pressure wave radiated at the outlet of the vent is in phase opposition to the pressure wave radiated into the enclosure by the diaphragm. The pressure wave radiated at the vent outlet is thus in-phase with the pressure wave radiated by the face of the diaphragm that faces towards the outside of the enclosure, such that the effects of the two pressure waves add and increase the power played-back sound.
It is known that such a resonator possesses a characteristic frequency representing a lower limit for the frequency of sounds that can be transmitted by the resonator. This characteristic frequency varies directly as a function of the section of the vent and inversely as a function of the volume of the enclosure and of the length of the vent.
In order to lower the characteristic frequency of the resonator and thus enable it to transmit sounds at very low frequency, it is necessary either to increase the volume of the enclosure, which then becomes bulky, or else to increase the length of the vent, which makes it difficult to position within the enclosure, or even to reduce the section of the vent. However, when the section is reduced, it is found that the sound power of the emitter decreases.